The 17-year-old had already set a new world mark in the 400m Freestyle - S6 en route to a successful defence of her Paralympic title on Saturday.
And she produced a blistering freestyle leg to touch in 3:06.97 this morning, 1.13 inside her own world mark from March.
Simmonds said afterwards: 'I wasn't going for a world record, I was actually going for the Paralympic record, but I was just going in there to feel the stroke.
'When I walk out the crowd is amazing, that definitely gives me a big buzz. I just can't wait to race tonight.
'I am quite shocked with that time. When I was at the finish I couldn't actually see what (the time) was, I had to ask someone.
'I'm really pleased with a two-second PB in the heats. It's good to be on top form at the biggest Paralympic Games of my life.'
China's Yang Yang also set a new world mark of 2:10.47 in the men's 100m Freestyle - S2, Ihar Boki of Belarus headed the 100m Backstroke - S13 in a record 58.19 while Russian Denis Tarasov set a new mark of 25.92 in the 50m Freestyle - S8.
Ukraine's Olga Sviderska reduced the 100m Freestyle - S3 mark to 1:45.34, Sweden's Maja Reichard dipped under her own 100m Breaststroke - SB11 mark (1:28.51) and Jacqueline Freney of Australia set a new Paralympic record of 1:09.74 in the 100m Freestyle - S7.